Correct spelling, explaination: this form is correct, because the last number pronounced here is first which ends with st. Similarly 55th is pronounced fifty-fifth and 63rd – sixty-third. Thirty-first has got st at the end, which is added to a number form as an abbreviation. This is the reason why 31st is the correct form, and not
Definition of 31st:
numeral, ordinal number between 30th and 32nd
He got a trip to Madrid for his 31st birthday.
Incorrect spelling, explaination: writing
31th is incorrect, because in the full-word form this number is pronounced as thirsty-first so at the end there is st which should become an abbreviation at the end of number form. This way we know that the correct form is 31st.
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